From iPhone App Buckingham Palace

I was playing a game on my iPhone called Buckingham Palace and in this mystery type game they had a puzzle. It is a rather easy one but thought I'd share it here. If you want to see the missing piece and just solve it then go HERE

Additions: When you place the piece it is not check, the piece you place cannot move, and then you still have to move a piece that is on the board before your placed the piece back on the board.